The present invention relates to an image forming apparatus, such as an electronic copying machine, and an image forming method for reading an image on an original and forming an image.
A 4-series tandem type image forming apparatus having four photosensitive drums is known as a conventional color electronic copying machine.
There are two types of 4-series tandem type image forming apparatuses: a type (direct transfer type) in which toner images on photosensitive drums are directly transferred onto paper, and an intermediate transfer type in which four-color toner images are first transferred (primary transfer) onto a semi-conductive intermediate transfer body, and then batch-transferred (secondary transfer) on paper.
In the primary transfer, toner on the photosensitive drum is transferred onto the intermediate transfer body by a transfer electric field occurring between a photosensitive drum and a transfer roller. If the transfer electric field is too small, toner fails to be transferred onto the intermediate transfer body, and a great amount of toner would remain on the photosensitive drum (incomplete transfer with residual toner). On the other hand, if the transfer electric field is too large, a Paschen-discharge occurs at an air gap portion near a transfer nip portion (transfer contact portion). The Paschen-discharge causes such a phenomenon (reverse transfer) that toner once transferred on a transfer medium is attracted back to the photosensitive drum. Thus, such contradictory problems will occur due to the transfer electric field.
Further, there is such a problem that part of the toner on the photosensitive drum scatters onto the transfer medium along the transfer electric field in the gap portion in the vicinity of the transfer nip (front-side transfer, rear-side transfer), resulting in image-quality degradation. How to suppress these phenomena is a great task.
In an image forming apparatus of a photoconductor-cleanerless type in which a photosensitive drum is not equipped with a cleaner, toner (residual transfer toner) that is not transferred on a transfer medium and remains on the photosensitive drum can be recovered in a developing device, and the amount of waste toner can be reduced. Moreover, the life of the photosensitive drum can advantageously be increased.
However, there is such a serious problem that if a reverse transfer phenomenon of plural-color toners occurs at the same time, toners in the developing device would be mixed.
All of these problems occur because the transfer electric field in the vicinity of the transfer nip is not completely controlled.